The thing is, every beginner lockpicker makes a similar point when they realize how easy most locks are: "what's the point of locking my door if anyone can easily get in anyway?".

I think the same answers apply here: because making it harder to be casually recorded sends a clear signal that you don't want it, and now the act of recording goes from being an oversight to a deliberate, sometimes punishable act.

I think your point is a little black-and-white — there's tons of behaviour that sits in the "technical possible but frowned upon" bucket.

It's like people listening to music without any headphones on the train — technically has been possible for ages but previously would've gotten you told to turn it off. Now it barely gets a raised eyebrow.

Can you prevent people secretly filming you? No, but most people still don't want it be become accepted behaviour, even if to you that's all just "whining and blathering".

So if someone wants to sucker punch me in pubic, there's also nothing that I or anyone else can do to proactively prevent it.

But I don't get sucker punched very often, so it seems like there probably are things that can be done about. Norms, consequences, etc etc. "We live in a society".